This invention relates to a logic circuit and more particularly to a contactless logic circuit equivalent to a relay contact circuit, which can be connected on the basis of a circuit diagram similar to the connections of relay contacts.
In carrying out a certain sequential control, a conventional system often uses relays which rely on mechanical closing or opening of relay contacts. This system has several defects such as the occurrence of frequent relay failures, short operating life, and the need for continual maintenance and inspection services. In recent years, there has been a tendency for relay contact systems to be gradually replaced by contactless electronic logic circuits employing semiconductor elements, which have functions equivalent to those of the conventional relay contact system.
In rewriting a control sequence to carry out the operation equivalent to that of a relay contact circuit in terms of a conventional logic circuit, there arises the need for employing particular logical circuit symbols which are entirely different from the symbols used in circuits using relays. Consequently, engineers or field operators who have become conversant with the conventional relay contacts symbols are confronted with the necessity of acquiring knowledge of entirely new and unfamiliar logical symbols and of their interpretations.
In translating a control sequence for a conventional relay contact circuit into that for a logic circuit, a conventional circuit diagram must be rewritten into an entirely new and unique one. In translating, the risk of errors in the sequential composition in quite conceivable.
Even if a circuit for realizing a certain sequence has been composed with a conventional logic circuit, such a logic circuit will have the defect of poor operational reliability due to noise that may be introduced into the circuit from the outside.
Various circumstances, as above mentioned, have been great obstacles for the transfer of the conventional logic system containing mechanical contacts into the contactless logic system.
Further, where a contactless control sequence is to be set up by use of conventional logic circuit boards, there arises the necessity of preparing at least three kinds of different boards, namely, AND element boards, OR element boards, and inverter element boards. These circuit boards must be mutually interconnected. The need for preparation of numerous kinds of boards invariably makes the circuit structure complex and space requirements become bulkier.